Well logging generally involves the generation and transmission of acoustic signals through an earth formation of interest, and the reception of the signals at a point spaced from the transmitter. By knowing the distance between the transmitter and the receiver, and the time required for an acoustic signal to travel that distance, the velocity of sound in the formation may be calculated. Once the velocity is known, other properties of the formation may be determined.
A problem encountered in acoustic well logging is that acoustic energy propagates in both shear and compressional modes, and the velocity of each mode depends on the propagating medium. Thus, the generation of a non-directional acoustic signal results in compressional waveforms in the drilling mud, and both shear and compressional waveforms in the borehole casing surrounding an earth formation, with each waveform having a different velocity. A composite waveform thus may be detected which overshadows the formation mode, and makes difficult an accurate representation of the time of arrival of the formation mode.
Attempts to overcome this problem have involved complicated analytical techniques as disclosed in U.S. Pat. No. 4,951,267; or complicated electronic and mechanical systems as disclosed in U.S. Pat. No. 3,475,722, U.S. Pat. No. 3,426,865, and U.S. Pat. No. Re 33,472.
A further problem arises from the seismic waves being reflected by a subterranean reflecting surface ("reflector"), and the reflected wave being detected by a conventional seismic detector such as a hydrophone as disclosed in U.S. Pat. No. 4,789,968. The monopole tube waves, and dipole and higher multipole order modal waves, tend to dominate the response and obscure the seismic waves that were reflected by the reflector.
Geophones attached directly to a borehole wall, as disclosed in U.S. Pat. No. 5,128,898, have been used instead of hydrophones to reduce such tube wave noise, but the speed at which down-hole data may be acquired is substantially reduced.
Further, directionally sensitive detectors have been used which are insensitive to all components of the seismic waves, except the component occurring in a particular direction. By way of example, only the component of the seismic wave impinging the detector in a line parallel to the longitudinal axis of the detector may be detected. The directionally sensitive detectors have been mounted for rotational movement on complex servo systems for orientation in a desired direction as disclosed in U.S. Pat. No. 2,959,240 and U.S. Pat. No. 3,496,533, or have required complex electrical and mechanical transducer structures as disclosed in U.S. Pat. No. 4,951,267.
Further shortcomings of the prior art appear in U.S. Pat. No. 3,961,307 which requires a planar surface of sufficient size to mount transducers, approximately a half wavelength. A logging tool is typically 4" in diameter. The wavelength of sound at 1000 Hz is 5 feet. Thus, the process disclosed in the patent is practical only at very high frequencies, which places a substantial limitation on the depth of investigation in an azimuthal direction.
The devices disclosed in U.S. Pat. Nos. 4,832,148 and 4,951,267 measure formation anisotropy, and will not work with isotropic formations. Neither of the devices will respond to objects such as salt domes or other boreholes in a formation.
U.S. Pat. No. 4,703,459 uses only a two-transducer array, and is sensitive to external noise sources such as gas leaks. No consideration is given to using a transmitter in conjunction with a receiver to allow echo ranging, and to determine the azimuth position of passive features such as the boundary of a reflecting subsurface formation of a salt dome, or another borehole. The two-transducer array that is disclosed would be too inaccurate to be useful for determining formation layer boundaries in a longitudinal borehole. That is, the signal that is produced is small at null, and has two broad maximum signals with little phase difference at the peaks. Further, the disclosed two-transducer array would be susceptible to noise in a direction ninety degrees to a principal response direction.
In accordance with the present invention, the direction of a transmitted waveform is selected by controlling the magnitude and polarity of the driving signal of an acoustic transmitter, and by detecting only that signal that is in the vector direction of a directionally sensitive receiver. The components of the received waveform thereby are effectively separated. When a three-element configuration of a monopole transmitter and a monopole/dipole receiver pair is used, a response having a single, unambiguous peak in the direction of a reflector is produced. Interfering signals such as reflections from fractures and other unwanted responses are discriminated against. The receiver pair is positioned so that a phase measurement is made when both the monopole and dipole receivers have substantial amplitudes. That is, one receiver is not looking away from the source.